Solar cell interconnection

ABSTRACT

A solar cell can include a conductive foil having a first portion with a first yield strength coupled to a semiconductor region of the solar cell. The solar cell can be interconnected with another solar cell via an interconnect structure that includes a second portion of the conductive foil, with the interconnect structure having a second yield strength greater than the first yield strength.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/496,226, filed on Sep. 25, 2014, the entire contents of which arehereby incorporated by reference herein.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of, and entering into, the substrate creates electron andhole pairs in the bulk of the substrate. The electron and hole pairsmigrate to p-doped and n-doped regions in the substrate, therebygenerating a voltage differential between the doped regions. The dopedregions are connected to conductive regions on the solar cell to directan electrical current from the cell to an external circuit.

Solar cells can be interconnected together in series to provide a stringof solar cells, which in turn can be connected in series to form amodule. In some instances, interconnecting solar cells can bechallenging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 2A illustrate cross-sectional views of a portion of examplesolar cells having conductive contacts formed on emitter regions formedabove a substrate, according to some embodiments.

FIGS. 1B and 2B illustrate cross-sectional views of a portion of examplesolar cells having conductive contacts formed on emitter regions formedin a substrate, according to some embodiments.

FIGS. 3-6 illustrate cross-sectional views of various example solar cellinterconnections, according to some embodiments.

FIGS. 7 and 8 illustrate top-down views of various example solar cellinterconnections, according to some embodiments.

FIG. 9 is a flowchart illustrating an example method of interconnectingsolar cells, according to some embodiments.

FIG. 10 is a flowchart illustrating an example method of forming regionsof conductive foil having different yield strengths, according to someembodiments.

FIGS. 11-13 illustrate cross-sectional views of an example sequence forforming regions of conductive foil having different yield strengths,according to some embodiments.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter of theapplication or uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. §112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, reference to a“first” portion of a conductive foil does not necessarily imply thatthis portion is the first portion in a sequence; instead the term“first” is used to differentiate this portion from another portion(e.g., a “second” portion).

“Based On.” As used herein, this term is used to describe one or morefactors that affect a determination. This term does not forecloseadditional factors that may affect a determination. That is, adetermination may be solely based on those factors or based, at least inpart, on those factors. Consider the phrase “determine A based on B.”While B may be a factor that affects the determination of A, such aphrase does not foreclose the determination of A from also being basedon C. In other instances, A may be determined based solely on B.

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

“Inhibit”—As used herein, inhibit is used to describe a reducing orminimizing effect. When a component or feature is described asinhibiting an action, motion, or condition it may completely prevent theresult or outcome or future state completely. Additionally, “inhibit”can also refer to a reduction or lessening of the outcome, performance,and/or effect which might otherwise occur. Accordingly, when acomponent, element, or feature is referred to as inhibiting a result orstate, it need not completely prevent or eliminate the result or state.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper”, “lower”, “above”,and “below” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

Although many of the examples described herein are back contact solarcells, the techniques and structures apply equally to other (e.g., frontcontact) solar cells as well. Moreover, although much of the disclosureis described in terms of solar cells for ease of understanding, thedisclosed techniques and structures apply equally to other semiconductorstructures (e.g., silicon wafers generally).

Solar cell interconnects and methods of forming solar cell interconnectsare described herein. In the following description, numerous specificdetails are set forth, such as specific process flow operations, inorder to provide a thorough understanding of embodiments of the presentdisclosure. It will be apparent to one skilled in the art thatembodiments of the present disclosure may be practiced without thesespecific details. In other instances, well-known fabrication techniques,such as lithography techniques, are not described in detail in order tonot unnecessarily obscure embodiments of the present disclosure.Furthermore, it is to be understood that the various embodiments shownin the figures are illustrative representations and are not necessarilydrawn to scale.

This specification first describes example solar cells that can beinterconnected with the disclosed interconnects, followed by a moredetailed explanation of various embodiments of interconnect structures.The specification then includes description of example methods forforming the interconnect structures. Various examples are providedthroughout.

In a first example solar cell, a conductive foil is used to fabricatecontacts, such as back-side contacts, for a solar cell having emitterregions formed above a substrate of the solar cell. For example, FIG. 1Aillustrates a cross-sectional view of a portion of a solar cell havingconductive contacts formed on emitter regions formed above a substrate,in accordance with an embodiment of the present disclosure. In variousembodiments, the conductive foil is also used to form an interconnectstructure that has a higher yield strength than the conductive foil ofthe conductive contacts, as described in more detail below.

Some challenges exist in coupling conductive foil to the solar cell andto interconnect conductive foil of adjacent cells. As one example,ratcheting can occur which can reduce reliability and lifetime of asolar cell and modules. Ratcheting is a form of plastic deformation ofmetal that is characterized by non-planar distortion of the foil, whichcan lead to reliability issues in the field. As another example, waferbowing can occur due to thermal stress mismatch between materials (e.g.,between silicon and metal) and can cause trouble with processing (e.g.,alignment) and handling. The relationship between the yield stress ofthe metal and the impact on both ratcheting and bowing is opposite. Forinstance, high yield stress metal can be good for ratcheting but bad forbow. The inverse is true for low yield stress metal. The disclosedstructures and techniques can inhibit wafer bowing and ratcheting andresult in improved lifetime and performance of the resulting solar cellsand modules.

Referring to FIG. 1A, a portion of solar cell 100A includes patterneddielectric layer 224 disposed above a plurality of n-type dopedpolysilicon regions 220, a plurality of p-type doped polysilicon regions222, and on portions of substrate 200 exposed by trenches 216.Conductive contacts 228 are disposed in a plurality of contact openingsdisposed in dielectric layer 224 and are coupled to the plurality ofn-type doped polysilicon regions 220 and to the plurality of p-typedoped polysilicon regions 222.

In one embodiment, the plurality of n-type doped polysilicon regions 220and the plurality of p-type doped polysilicon regions 222 can provideemitter regions for solar cell 100A. Thus, in an embodiment, conductivecontacts 228 are disposed on the emitter regions. In an embodiment,conductive contacts 228 are back contacts for a back-contact solar celland are situated on a surface of the solar cell opposing a lightreceiving surface (direction provided as 201 in FIG. 1A) of solar cell100A. Furthermore, in one embodiment, the emitter regions are formed ona thin or tunnel dielectric layer 202.

In some embodiments, as shown in FIG. 1A, fabricating a back-contactsolar cell can include forming thin dielectric layer 202 on thesubstrate. In one embodiment, a thin dielectric layer is composed ofsilicon dioxide and has a thickness approximately in the range of 5-50Angstroms. In one embodiment, thin dielectric layer performs as a tunneloxide layer. In an embodiment, the substrate is a bulk monocrystallinesilicon substrate, such as an n-type doped monocrystalline siliconsubstrate. However, in another embodiment, the substrate includes apolycrystalline silicon layer disposed on a global solar cell substrate.

Trenches 216 can be formed between n-type doped polysilicon (oramorphous silicon) regions 220 and p-type doped polysilicon regions 222.Portions of trenches 216 can be texturized to have textured features.Dielectric layer 224 can be formed above the plurality of n-type dopedpolysilicon regions 220, the plurality of p-type doped polysiliconregions 222, and the portions of substrate 200 exposed by trenches 216.In one embodiment, a lower surface of dielectric layer 224 can be formedconformal with the plurality of n-type doped polysilicon regions 220,the plurality of p-type doped polysilicon regions 222, and the exposedportions of substrate 200, while an upper surface of dielectric layer224 is substantially flat. In a specific embodiment, the dielectriclayer 224 is an anti-reflective coating (ARC) layer.

A plurality of contact openings can be formed in dielectric layer 224.The plurality of contact openings can provide exposure to the pluralityof n-type doped polysilicon regions 220 and to the plurality of p-typedoped polysilicon regions 222. In one embodiment, the plurality ofcontact openings is formed by laser ablation. In one embodiment, thecontact openings to the n-type doped polysilicon regions 220 havesubstantially the same height as the contact openings to the p-typedoped polysilicon regions 222.

Forming contacts for the back-contact solar cell can include formingconductive contacts 228 in the plurality of contact openings 226 andcoupled to the plurality of n-type doped polysilicon regions 220 and tothe plurality of p-type doped polysilicon regions 222. Thus, in anembodiment, conductive contacts 228 are formed on or above a surface ofa bulk N-type silicon substrate 200 opposing a light receiving surface201 of the bulk N-type silicon substrate 200. In a specific embodiment,the conductive contacts are formed on regions (222/220) above thesurface of the substrate 200.

Still referring to FIG. 1A, conductive contacts 228 can include aconductive foil 134. In various embodiments, conductive foil can bealuminum, copper, other conductive materials, and/or a combinationthereof. In some embodiments, as shown in FIG. 1A, conductive contacts228 can also include one or more conductive (metal or otherwise)regions, such as regions 130 and 132 in FIG. 1A, between conductive foil134 and a respective semiconductor region. For example, a firstconductive region 130 can include (e.g., aluminum, aluminum/siliconalloy, etc.), which can be printed, or blanket deposited (e.g.,sputtered, evaporated, etc.).

In one embodiment, the conductive foil 134 and the one or moreconductive regions 130 and 132 can be welded, thermally compressed, orotherwise coupled to the semiconductor region of the solar cell andtherefore in electrical contact with the emitter regions of the solarcell 100A. As described herein, in some embodiments, as shown in FIGS.1A and 1B, one or more conductive regions (e.g., sputtered, evaporated,or printed aluminum, nickel, copper, etc.) may exist between theconductive foil and the emitter regions. Thermally compressed conductivefoil is used herein to refer to the a conductive foil that has beenheated at a temperature at which plastic deformation can occur and towhich mechanically pressure has been applied with sufficient force suchthat the foil can more readily adhere to the emitter regions and/orconductive regions.

In some embodiments, the conductive foil 134 can be aluminum (Al) foil,whether as pure Al or as an alloy (e.g., Al/Silicon (Al/Si) alloy foil).In one embodiment, the conductive foil 134 can also include non-Almetal. Such non-Al metal can be used in combination with or instead ofAl particles. Although much of the disclosure describes metal foil andmetal conductive regions, note that in some embodiments, non-metalconductive foil (e.g., conductive carbon) and non-metal conductiveregions can similarly be used in addition to or instead of metal foiland metal conductive regions. As described herein, metal foil caninclude Al, Al—Si alloy, tin, copper, and/or silver, among otherexamples. In some embodiments, conductive foil can be less than 5microns thick (e.g., less than 1 micron), while in other embodiments,the foil can be other thicknesses (e.g., 15 microns, 25 microns, 37microns, less than 50 microns, etc.) In some embodiments, the type offoil (e.g., aluminum, copper, tin, etc.) can influence the thickness offoil needed to achieve sufficient current transport across the solarcell. Moreover, in embodiments having one or more additional conductiveregions 130 and 132, the foil can be thinner than in embodiments nothaving those additional conductive regions.

Moreover, in various embodiments, the type and/or thickness of theconductive foil can affect the yield strength of the portion of theconductive foil coupled to the solar cell and the portion of theconductive foil that overhangs the edge of the solar cell and is part ofthe interconnect structure.

In various embodiments, conductive regions 130 and 132 can be formedfrom a metal paste (e.g., a paste that includes the metal particles aswell as a binder such that the paste is printable), from a metal powder(e.g., metal particles without a binder, a powder of Al particles, alayer of Al particles and a layer of Cu particles), or from acombination of metal paste and metal powder. In one embodiment usingmetal paste, paste may be applied by printing (e.g., screen printing,ink-jet printing, etc.) paste on the substrate. The paste may include asolvent for ease of delivery of the paste and may also include otherelements, such as binders or glass frit.

In various embodiments, the metal particles can be fired (before and/orafter the conductive foil and conductive regions are coupled together),also referred to as sintering, to coalesce the metal particles together,which can enhance conductivity and reduce line resistance therebyimproving the performance of the solar cell. But heating from firing orthe bonding process can also reduce the yield strength of the conductivefoil, which can reduce reliability and lifetime of the solar module fromratcheting. Accordingly, techniques and structures disclosed herein canprovide for a sufficiently low yield strength for the conductive foilover the solar cell to inhibit bowing yet also provide for asufficiently high yield strength of the foil of the interconnectstructure so as to inhibit ratcheting.

Turning now to FIG. 1B, a cross-sectional view of a portion of anexample solar cell having conductive contacts formed on emitter regionsformed in a substrate is illustrated, according to one embodiment. Forexample, in this second exemplary cell and similar to the example ofFIG. 1A, conductive foil can be used to fabricate contacts, such asback-side contacts, for a solar cell having emitter regions formed in asubstrate of the solar cell.

As shown in FIG. 1B, a portion of solar cell 100B includes patterneddielectric layer 124 disposed above a plurality of n-type dopeddiffusion regions 120, a plurality of p-type doped diffusion regions122, and on portions of substrate 100, such as a bulk crystallinesilicon substrate. Conductive contacts 128 are disposed in a pluralityof contact openings disposed in dielectric layer 124 and are coupled tothe plurality of n-type doped diffusion regions 120 and to the pluralityof p-type doped diffusion regions 122. In an embodiment, diffusionregions 120 and 122 are formed by doping regions of a silicon substratewith n-type dopants and p-type dopants, respectively. Furthermore, theplurality of n-type doped diffusion regions 120 and the plurality ofp-type doped diffusion regions 122 can, in one embodiment, provideemitter regions for solar cell 100B. Thus, in an embodiment, conductivecontacts 128 are disposed on the emitter regions. In an embodiment,conductive contacts 128 are back contacts for a back-contact solar celland are situated on a surface of the solar cell opposing a lightreceiving surface, such as opposing a texturized light receiving surface101, as depicted in FIG. 1B.

In one embodiment, referring again to FIG. 1B and similar to that ofFIG. 1A, conductive contacts 128 can include a conductive foil 134 andin some embodiments, one or more additional conductive regions, such asconductive regions 130 and 132. The conductive foil 134, and the one ormore conductive regions can be coupled (e.g., welded, thermallycompressed, or otherwise) to the semiconductor region of the solar celland/or to one or more conductive regions between the foil and thesemiconductor region and therefore in electrical contact with theemitter regions of the solar cell 100A. The conductive contactdescription of FIG. 1A applies equally to the conductive contact of FIG.1B but is not repeated for clarity of description.

Turning now to FIG. 2A, the illustrated solar cell includes the samefeatures as the solar cell of FIG. 1A except that the example solar cellof FIG. 2A does not include the one or more additional conductiveregions (regions 130 and 132 of FIG. 1A). Instead, conductive foil 134is bonded directly to the semiconductor region of the solar cell.

Similarly, the illustrated solar cell of FIG. 2B includes the samefeatures as the solar cell of FIG. 1B except that the example solar cellof FIG. 2B does not include the one or more additional conductiveregions (regions 130 and 132 of FIG. 1B). Instead, conductive foil 134is bonded directly to the semiconductor region of the solar cell.

Although certain materials are described herein, some materials may bereadily substituted with others with other such embodiments remainingwithin the spirit and scope of embodiments of the present disclosure.For example, in an embodiment, a different material substrate, such as agroup III-V material substrate, can be used instead of a siliconsubstrate.

Note that, in various embodiments, the formed contacts need not beformed directly on a bulk substrate, as was described in FIGS. 1B and2B. For example, in one embodiment, conductive contacts such as thosedescribed above are formed on semiconducting regions formed above (e.g.,on a back side of) as bulk substrate, as was described for FIGS. 1A and2A.

In various embodiments, the conductive foil of solar cells of FIGS.1A-1B and 2A-2B includes an overhang region (e.g., tab) that extendsbeyond the edge of the cell and can be coupled to an overhang region ofan adjacent cell to interconnect the two cells together. In someembodiments, the overhang portion of a particular solar cell can extendless than 2 mm over its edge.

Turning now to FIGS. 3-8, various examples of solar cell interconnectstructures configured to inhibit ratcheting and wafer bowing areillustrated.

FIG. 3 illustrates two solar cells, solar cells 300 a and 300 b, coupledtogether via an interconnect structure. In the illustrated example, aportion 302 a of a conductive foil is coupled to solar cell 300 a and aportion 302 b of another conductive foil is coupled to solar cell 300 b.The interconnect structure can include overhang portions 304 a and 304 bof the conductive foils. As shown, the overhang portions 304 a and 304 bcan be coupled via one or more joints 306, which can be formed via laseror electrical welding, soldering, or some other technique. In variousembodiments, the portions of the conductive foils disposed above andcoupled to the solar cells have a lower yield stress than the yieldstress of the interconnect structure. Accordingly, the portion of theconductive foil that includes lower yield stress metal, which can helpinhibit wafer bowing, is the portion that is coupled to the wafer. Andthe portion of the foil that is used to form the interconnect structurecan be higher yield stress metal, which can inhibit ratcheting.Accordingly, such a foil can inhibit both wafer bowing and ratcheting.One example technique to form dual tempered conductive foil is describedat FIGS. 10-13.

FIG. 4 illustrates another example interconnect structure. Similar toFIG. 3, a portion 402 a of a conductive foil is coupled to solar cell400 a and a portion 402 b of another conductive foil is coupled to solarcell 400 b. In contrast to FIG. 3, the interconnect structure of FIG. 4includes an additional material 408 coupled to and between overhangportions 404 a and 404 b. In one embodiment, additional material 408 canbe a material such that the collective yield strength of theinterconnect structure is high enough to inhibit ratcheting. In oneembodiment, the conductive foils, including the portions disposed overand coupled to the solar cells and the overhang portions, have a loweryield strength to inhibit bowing. The additional material, however, canhave sufficiently high yield strength such that, when added to the loweryield strength foil of the overhang portions, the collectiveinterconnect structure has a high enough yield strength to inhibitratcheting.

In various embodiments in which additional material 408 is positionedbetween the overhang tabs, as in FIG. 4, additional material 408 is aconductive material. It can be the same material as the overhang tabs(e.g., soft aluminum overhang tabs, and hard aluminum additionalmaterial) or different.

FIG. 5 illustrates another example interconnect structure for use ininterconnecting solar cells. The interconnect structure is similar tothe interconnect structure of FIG. 4 except that the additional material508 is not between overhang portions 504 a and 504 b. Instead, in theexample of FIG. 5, additional material 508 is disposed between solarcells 500 a and 500 b, which can provide and/or maintain a consistentspacing or gap between the solar cells. For a back contact solar cell,additional material 508 is located on the sunny side of the interconnectstructure and can be visible from the sunny side of a solar module.Accordingly, in one embodiment, the additional material 508 can becolored or otherwise made such that the visible portion of theinterconnect structure, as viewed from the front of the module, is asimilar color to the solar cells and therefore blends in.

In some embodiments, additional material 508 can be a conductivematerial or in some instances, it can be a non-conductive material aslong as the additional material 508 can be coupled (e.g., welded,soldered, wrapped around, tied around, etc.) to the overhang tabs and aslong as the interconnect structure collectively has sufficient yieldstrength to inhibit ratcheting.

Various other examples also exist. For example, in one embodiment,instead of the additional material being on the front side of theoverhang tabs or between the overhang tabs, the additional material canbe on the back side of the overhang tabs. In another embodiment, theadditional material can be wrapped around the overhang tabs and thencoupled to the overhang tabs to form the interconnect structure.

In the examples of FIGS. 4 and 5, the additional material is shown at anon-zero angle relative to the solar cells. In some embodiments, theadditional material can be coupled such that the interconnect structureis slightly out of plane from the solar cell, which can result in strainrelief for the interconnect structure, thereby further inhibitingratcheting. Other examples of stress relief interconnect structures alsoexist. One such example is illustrated in FIG. 6. As shown in FIG. 6,additional material 608 includes a bend (e.g., c-shaped bend) such thatadditional material is coupled separately to each overhang tab. Such aninterconnect structure can result in improved strain relief and furtherinhibit ratcheting. Although the illustrated additional material shows atwo-axis bend, in some embodiments, the bend can be a three-axis bend.

The additional material of the interconnect structure can be a varietyof shapes. The interconnect structure can be a simple ribbon, a channelshape (e.g., for additional stiffness), a bowtie shape (for aestheticsand connection in the diamond areas of the module). The additionalmaterial can have other materials or properties to modify the joint orreliability. Such materials or properties include coating for corrosionprotection (e.g., metal, oxide, or nitride) or for use in coupling tofoil (e.g., solder coating on the additional material), adhesiveproperties for adhesion to module materials (e.g., encapsulant) or tothe overhang portions (e.g., solder material coating), or multiplelayers for different expansion and contraction properties.

FIGS. 7 and 8 illustrate example interconnected solar cells, accordingto various embodiments. For ease of explanation, the metal on solarcells 700 and 710 is not illustrated as patterned (e.g., fingerpattern). As shown, solar cells 700 and 710 are interconnected viamultiple interconnect structures 720 at the corners of the solar cells.The right-most dashed lines illustrate the edge of the overhang tab fromthe conductive foil of the left solar cell and the left-most dashedlines illustrate the edge of the overhang tab from the conductive foilof the right solar cell. In some embodiments, joint 730 can be a weldjoint, solder joint, or some other coupling, and can be the location atwhich the overlapping overhang tabs are coupled together.

As shown in the example of FIG. 8, instead of interconnecting the solarcells at their respective corners, the solar cells are connected at theoverlapping overhang edges 820 of the solar cells with a plurality ofinterconnect joints 830. In one embodiment, one or more of theinterconnect joints 830 can correspond to a separate piece of additionalmaterial, such as a separate piece of hard foil. Or, in one embodiment,a continuous piece of additional material can be above, between, orunder the overlapping edges and coupled to the overlapping overhangedges at the locations of interconnect joints 830. In some embodiments,however, such as in the embodiment of FIG. 3, no additional material isused. In such embodiments, joints 730 and 830 can simply be regions inwhich one overhang foil is coupled to another.

In one embodiment, one or more stress relief features can be added tothe interconnect structure after it has been formed. For example, in oneembodiment, one or more relief cutouts can be formed in the interconnectstructure to further relieve stress.

Turning now to FIG. 9, a flow chart illustrating a method for forming asolar cell interconnect region is shown, according to some embodiments.In various embodiments, the method of FIG. 9 may include additional (orfewer) blocks than illustrated. For example, in some embodiments,coupling an interconnect material to the overhang tabs, as shown atblock 908, may not be performed.

As shown at 902, a portion of a conductive foil can be coupled to asolar cell. For example, in one embodiment, a portion of the conductivefoil disposed over the solar cell can be coupled to a semiconductorregion of the solar cell. Coupling can be achieved by laser or thermalwelding, soldering, thermocompression, among other techniques.

As illustrated at 904, a portion of another conductive foil can becoupled to another solar cell. Similar to the description at block 902,in one embodiment, a portion of the other conductive foil disposed overthe other solar cell can be coupled to a semiconductor region of theother solar cell. As was the case with block 902, coupling can beachieved by laser or thermal welding, soldering, thermocompression,among other techniques. In various embodiments, blocks 902 and 904 canbe performed sequentially or can be processed at substantially the sametime.

At 906, other portions of the conductive foils can be coupled togetherto form an interconnect structure. In one embodiment, the other portionsare overhang portions that extend past the edge of the solar cells.Various examples are illustrated in FIGS. 3-8. The overhang portions canat least partially overlap and the overlapping regions can be welded,soldered, or otherwise coupled together such that the cells areelectrically and mechanically interconnected together.

In some embodiments, the overhang portion of the foils can have higheryield strength than the portion of the foil disposed over and coupled tothe solar cell such that wafer bowing and ratcheting can be inhibited.In one embodiment, the foil can be fabricated or modified to be dualtempered, such that the overhang portion is a hard foil and the solarcell portion is a soft foil. FIGS. 10-13 illustrate one exampleembodiment or modifying the foil to be dual tempered.

In some embodiments, however, an additional material can be coupled tothe overhang foil portions to form the interconnect having the higheryield strength as illustrated at 908. For example, in one embodiment,the additional material can be placed between the overhang foil portionsor on the front or back side of the overhang foil portions and theadditional material, and both overhang foil portions can be coupledtogether to collectively form the interconnect. As one simple example,the overhang portions may be the same lower yield strength, soft foil asthe solar cell portions of the foil but the additional material may havehigh enough yield strength to make the overall interconnect structurehave a sufficiently high yield strength to inhibit ratcheting.

In one embodiment, the coupling of the two overhang portions and thecoupling of the additional material at block 906 and 908 can beperformed substantially simultaneously, or block 906 can be performedfirst, or block 908 can be performed first. As one example, theadditional material can be welded to one of the overhang portions first,and then the other overhang portion can be welded to the already weldedoverhang portion and additional material. Other variations also exist.

In some embodiments, the additional material is conductive whereas inother embodiments, the additional material may not be conductive or maynot be as conductive as the foil. In such embodiments, the overhangportions may make direct contact to one another without the additionalmaterial being between the overhang portions. In such embodiments, theadditional material can provide mechanical integrity and allow forsufficient yield strength to inhibit ratcheting but may not be reliedupon to carry current from one cell to the other.

Turning now to FIG. 10, a flow chart illustrating a method for forming adual tempered conductive foil is shown, according to some embodiments.In various embodiments, the method of FIG. 10 may include additional (orfewer) blocks than illustrated.

As shown at 1002, a hot bonding technique can be performed to couple aportion of a conductive foil to a solar cell. Similar to blocks 902 and904 of FIG. 9, in one embodiment, a portion of the conductive foildisposed over the solar cell can be coupled to a semiconductor region ofthe solar cell. Hot bonding techniques include thermocompression bondingand heated welding. For thermocompression bonding, the conductive foilcan be heated to temperatures above 200 degrees Celsius and mechanicalforce (e.g., via plate, roller, etc.) can be applied with pressure of atleast 1 psi.

In one embodiment, the conductive foil used in the method of FIG. 10 isa hard foil with high yield strength before the hot bonding technique isapplied. One example hard foil is 7020 Series Aluminum foil but otherhardness Al foils or other non-Al foils can be used.

As illustrated at 1004, a second portion of the conductive foil, whichcan be a portion that corresponds to overhang portions that extend pastthe edge of the solar cell, can be cooled during the hot bondingtechnique of block 1002. The result of blocks 1002 and 1004 is that theoriginal hard, higher yield strength conductive foil is softened andmodified into a lower yield strength foil in the portion over the solarcell as it is coupled to the solar cell to inhibit wafer bowing yetsubstantially retains its hardness in the overhang portions of the foilto inhibit ratcheting once interconnected.

Clamping the overhang portions to cool them can be difficult, especiallywhen the overhang portions may only extend 2 mm or less past the edge ofthe solar cell. In one embodiment, a larger overhang portion may existduring the dual tempering process and the overhang portion may then betrimmed after the foil has been tempered. For example, during the dualtempering process, the overhang portions may extend about 10 mm past theedges of the wafer such that the overhang portion is sufficiently enoughfor the clamp to hold the overhang portion. After the dual temperingprocess, the overhang portions can then be trimmed to a small length(e.g., 2 mm long, 1 mm, etc.).

FIGS. 11-13 illustrate cross-sectional views of portions of an exampledual tempering technique. As shown in FIG. 11, wafer 1102 can be locatedon a surface 1100, such as a wafer chuck. In one embodiment, surface1100 can be heated during the process to help the foil bond to the solarcell. Hard foil 1104 can be placed on wafer 1102 (e.g., on a backside ofthe wafer for a back contact solar cell) and clamped via clamps 1106 aand 1106 b. Not illustrated, the foil can be pressed, vacuumed, orotherwise be mechanically held in place and sufficiently taut.

FIG. 12 illustrates a hot bond 1108 being applied to the portion of thefoil over the solar cell. At the same time, clamps 1106 a and 1106 b canbe cooled (e.g., air chilled, water or coolant chilled, etc.) such thatthe heat from hot bond 1108 does not transfer enough to the overhangportions of the foil to lower the yield strength of the overhangportions.

Instead, as shown at FIG. 13, the resulting foil includes a lower yieldstrength portion 1110 disposed over and bonded to the solar cell 1102and higher yield strength overhang portions 1112 a and 1112 b, each ofwhich can be coupled to an overhang portion of a respective adjacentsolar cell to electrically interconnect the cells but also be stiffenough to inhibit ratcheting. In one embodiment, the interconnectstructure can simply be the coupled together higher yield strengthoverhang portions or it can also include the additional material asdescribed herein.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

The invention claimed is:
 1. A photovoltaic (“PV”) module, comprising: a first solar cell that includes a first conductive foil, wherein the first conductive foil is a single layer of material having a first portion with a first yield strength and a second portion with a second yield strength, wherein the first portion is disposed above and coupled to a semiconductor region of the first solar cell and wherein the second portion extends past an edge of the first solar cell; and an interconnect structure coupled to a second solar cell, wherein the interconnect structure includes the second portion; wherein the second yield strength is different than the first yield strength.
 2. The PV module of claim 1: wherein the second solar cell includes a second conductive foil having a third portion and a fourth portion, wherein the third portion is disposed above and coupled to a semiconductor region of the second solar cell, and wherein the interconnect structure includes the second portion of the first conductive foil coupled to the fourth portion of the second conductive foil.
 3. The PV module of claim 1, wherein the first conductive foil has a thickness of less than or equal to 50 microns.
 4. The PV module of claim 1, wherein the second portion extends less than 2 mm past the edge of the first solar cell.
 5. The PV module of claim 1, wherein the first conductive foil includes aluminum.
 6. The PV module of claim 1, further comprising: a gap between the first solar cell and the second solar cell.
 7. The PV module of claim 2, further comprising: a weld between the second portion of the first conductive foil and the fourth portion of the second conductive foil.
 8. A photovoltaic (“PV”) module, comprising: a solar cell that includes a conductive foil, wherein the conductive foil is a single layer of material having a first portion with a first yield strength and a second portion with a second yield strength, wherein the first portion is disposed above and coupled to a semiconductor region of the solar cell and wherein the second portion extends past an edge of the solar cell; and wherein the second yield strength is greater than the first yield strength.
 9. The PV module of claim 8, wherein the conductive foil has a thickness of less than or equal to 50 microns.
 10. The PV module of claim 8, wherein the second portion extends less than 2 mm past the edge of the solar cell.
 11. The PV module of claim 8, wherein the conductive foil includes aluminum. 